Standard 3 phase circuit can have different topologies, but most common are star (sometimes also called Y, T or wye) and delta topology (sometimes also called triangle or pi), both given in figure bellow.
Given 3 phase network can be transformed from delta to star and vice versa. Impedances between points A and B, B and C and C and A has to be preserved. Impedances are follows:
By solving above set of equations for ZA, ZB and ZC, star-delta (y-delta) transformation is obtained, and star-delta (y-delta) transformation calculation is:
With star-delta transformation, 3 phase circuit is transformed from star to delta topology with equivalent impedance.